This invention relates generally to semiconductor devices and more particularly to bipolar transistors.
As is known in the art, efficient generation of microwave power by solid state devices is required as an alternative to vacuum tube technology. Vacuum tubes such as travelling wave tubes, cross-field amplifiers and the like are costly and can be unreliable since high voltage power supplies and bias modulators are required. Solid state devices provide reliable, low voltage, low cost alternatives for many applications including, for example, phased array antennas and solid state transmitters.
One device used to generate microwave power is the heterojunction bipolar transistor (HBT). The HBT is similar to the conventional bipolar transistor in that the HBT includes collector, base and emitter layers disposed to form a pair of junctions. In general, a bipolar transistor is a three terminal device in which the upper layers (i.e. the base and emitter layers) are etched away in order to expose the underlying collector layer. Contacts are made to each of the layers to provide the three terminal device having collector, emitter, and base contacts. Generally, for an NPN type of device where the P material is the base layer, a hole current is injected into the base which produces in response an electron current across the emitter-base junction. If the hole current can be made relatively small in comparison to the emitter current which is produced across the emitter-based junction in response to the hole or base current, then the relatively small hole current can control a relatively large emitter current and the difference between the amount of hole current and the amount of emitter current produced will provide amplification.
In silicon device technology, p-type and n-type dopants, having relatively similar and relatively high hole and electron mobilities respectively, are available which has permitted the development of a practical bipolar transistor with the use of alternating conductivity-type doped silicon layers, (i.e. alternating n-type and p-type layers) P-type doped gallium arsenide layers, however, have significantly lower hole mobilities than the electron mobilities of N-type doped GaAs. The resulting high base resistance has prevented practical development of a bipolar NPN gallium arsenide transistor for microwave applications.
To overcome this problem, the heterojunction bipolar transistor (HBT) was conceived. The HBT differs from a conventional bipolar transistor in that the HBT incorporates an emitter material having a band gap larger than that of the material used in the base. This arrangement provides an abrupt energy discontinuity at the base-emitter junction This discontinuity acts as a barrier to hole current which permits the use of a substantially higher base doping than in a conventional bipolar transistor. This reduces the parasitic base resistance and the accompanying RC time constants. In turn, thinner base layers can be used for faster response times. HBT operation in X-band is relatively simple to obtain.
Maximum peak power of the HBT is determined by the area of the HBT device which can be effectively matched without gain reduction due to parasitic resistances in series with the intrinsic transistor. However, large area HBTs have the disadvantage of low input impedance in the common emitter or common base modes and low output impedance in the common collector mode.
As is generally known, a circuit approach to increasing power and efficiency is to utilize complimentary NPN and PNP transistor devices in the so-called "push-pull" configuration. In a so-called "Class B" push-pull configuration a bias voltage is supplied to the base contacts of complimentary HBTs such that with no RF signal, neither HBT conducts. Alternate half cycles of the RF input signal cause the transistors to conduct alternately, and the overall collector efficiency of the amplifier is theoretically 78.5%. If the transistors have similar DC terminal current voltage relationships, the required load impedance of the pair is the same as that for a single transistor. In a so-called push-pull common collector configuration the emitter voltage amplitude is limited to approximately the emitter-collector breakdown voltage for either transistor but the total current swing is twice that of a single transistor. Thus, the output power delivered to a load is double that of a single transistor.
HBTs in a push-pull circuit configuration have been fabricated as MMIC chips by growing two different transistor layers separated by a distance on a substrate. The HBTs are formed in the two separated layers and are connected with a transmission line. Several problems exist with this push-pull circuit structure. The first problem is the large size of the resulting circuit. Due to yield problems of monolithic microwave integrated circuits (MMICs), area on a wafer is very costly and thus it is desirable to maximize the number of circuits fabricated on a given wafer. A second problem is that the high frequency performance of the circuit is limited because of the required transmission line interconnect between the two transistors.
Thus it would be desirable to provide a push-pull HBT structure which takes advantage of the high peak power capability of the HBT, which is relatively efficient and which is compact in size and which is capable of high frequency operation.